The heater guys duly came, said they could see the problem, and gave the faulty valve a whack with a spanner. This fixed it, for at least 45 minutes. Fortunately, I had decided to splurge, and get the part replaced. It could take another week for the part to arrive, and for them to return. And it's cold.
Despite this, my garden has started to grow. The broad beans and snowpeas are erupting out of the mulch, and looking great, although I haven't inspected them in this rather heavy frost. The pansies are also flowering nicely.
Finally, some knitting. Oh, and some gratuitous nephew cuteness. The cardy is Knitty's Haiku in Cleckheaton Country Silk.